1. Field of the Invention
The present invention relates to a secondary battery, and, more particularly, to a secondary battery having an improved electrical connection structure of a secondary protective device which improves the stability of the secondary battery and reduces the inner space of the can.
2. Description of the Related Art
As portable wireless appliances including video cameras, mobile telephones, and portable computers generally become lighter while incorporating more functions, secondary batteries have been researched intensively for use as the driving power for the appliances. For example, secondary batteries include nickel cadmium batteries, nickel hydrogen batteries, nickel zinc batteries, and lithium secondary batteries. Lithium secondary batteries are widely used in the cutting-edge electronic appliance field because they are rechargeable. They can be made in a compact size with a large capacity. They have a high operation voltage. They also have a high energy density per unit weight.
In secondary batteries, an electrode assembly, which includes positive and negative electrode plates and a separator, is contained in a metallic can. An electrolyte is then injected into the can, which is sealed to form a bare cell. The bare cell generally has an electrode terminal positioned on the top thereof while being insulated from the can. The electrode terminal acts as one electrode of the battery and the battery can itself acts as the other electrode thereof.
After the bare cell is sealed, a safety apparatus including a secondary protective device, such as a positive temperature coefficient (PTC) thermistor, and a protective circuit module (PCM) is connected to the top of the bare cell and is encased in a battery pack or molded with a resin to form a secondary battery. The safety apparatus is connected to positive and negative electrodes and interrupts electrical currents when the temperature or voltage of the battery rises abruptly due to overcharge or over-discharge, in order to avoid dangers such as the fracture of the battery.
FIG. 1 is a partial sectional view showing a bare cell of a conventional secondary battery according to the prior art. The bare cell 10 of the secondary battery includes a can 20, an electrode assembly 22, and a cap assembly 30. The bare cell 10 may also include a secondary protective device 40 on the top thereof, depending on the construction of the secondary battery.
The can 20 is a metallic container having a hexahedron shape but with its top open. The can may be fabricated by deep drawing. The can 20 may be made of aluminum or an aluminum alloy, which is light metal of good conductivity and excellent corrosion resistance. The can 20 contains the electrode assembly 22, which includes a positive electrode 23, a separator 24, and a negative electrode 25, and an electrolyte. After the electrode assembly 22 is inserted into the can 20 through its top opening, the top opening is sealed by the cap assembly 30.
The cap assembly 30 is provided with a planar plate-type cap plate 31 having the size and shape corresponding to those of the top opening of the can 20.
The electrode assembly 22 is formed by winding positive and negative electrodes 23, 25 with a separator 24 interposed between them. The positive electrode 23 is electrically connected to the cap plate 31 via a positive electrode tab 26 and the negative electrode 25 to a negative terminal 32 of the cap plate 31 via a negative electrode tab 27. The can 20 is electrically insulated from the negative terminal 32 and acts as a positive terminal. After the cap assembly 30 is welded to the top of the can 20, an electrolyte is injected through an electrolyte injection hole 36 of the cap plate 31. The electrolyte injection hole 36 is sealed by a cap 37 made by press-fitting a ball.
The cap plate 31 may be made of the same material as that of the can 20, i.e., aluminum or an aluminum alloy, for improved welding to the can 20. The cap plate 31 has a terminal through-hole formed in the center thereof, through which the negative terminal 32 can pass. A tube-shaped gasket 33 is positioned on the exterior of the negative terminal 32 which extends through the center of the cap plate 31 for electrical insulation between the negative terminal 32 and the cap plate 31. An insulation plate 34 is positioned beneath the cap plate 31 near the terminal through-hole of the cap plate 31. A terminal plate 35 is positioned beneath the insulation plate 34.
The secondary protective device 40 is seated on the top surface of the cap plate 31 and is connected between the negative terminal 32 and a PCM (not shown in the drawing). A PTC thermistor or a thermal fuse may be used as the secondary protective device 40.
However, conventional secondary batteries configured as above have a problem in that the secondary protective device, which must detect the temperature change inside the can and interrupt electrical currents if necessary, is positioned on the exterior of the can and has a deteriorated temperature sensitivity.
Accordingly, there is a need for a secondary battery having an improved electrical connection in order to improve the stability of the secondary battery while minimizing the reduction in the inner space of the can.